1. Field of the Invention
The present invention relates to a device for controlling the heating of a catalyzer for purifying an exhaust gas.
2. Description of the Related Art
A catalyzer for purifying the exhaust gas is normally arranged in the exhaust passage of the engine, and when the engine is started, the temperature of the catalyzer is gradually increased due to the heating thereof by the exhaust gas. This exhaust gas purifying operation by the catalyzer, however, cannot be properly carried out until the catalyzer is activated, i.e., until the temperature of the catalyzer reaches an activation temperature, and accordingly, the exhaust gas cannot be purified by the catalyzer until the temperature of the catalyzer reaches the activation temperature thereof after the engine is started.
Therefore, in a known engine, an electric heater is provided for heating the catalyzer and is supplied with electric power for a predetermined time after the engine is started (see Japanese Unexamined Utility Model Publication No. 47-22313). In this engine, when the engine is started the catalyzer is rapidly heated, and accordingly, the temperature of the catalyzer rises rapidly to the activation temperature thereof, to thus purify the exhaust gas shortly after the engine is started.
If the catalyzer is used for a long time, however, and thus is deteriorated, the activation temperature at which the catalyzer starts the proper purifying operation of the exhaust gas becomes gradually higher as the catalyzer deteriorates. Nevertheless, as in the above-mentioned engine, where the electric heater is supplied with electric power for a predetermined time after the engine is started, the temperature of the catalyzer is increased only to a predetermined temperature, regardless of for how long the catalyzer is used. Accordingly, in this case, if the time for which the electric heater is supplied with electric power is determined such that the temperature of the catalyzer rises to the activation temperature when the catalyzer is new, the temperature of the catalyzer cannot be raised to the activation temperature before the supply of electric power to the electric heater is stopped, when the catalyzer has deteriorated. Accordingly, a problem arises in that, when the catalyzer has deteriorated, the exhaust gas cannot be purified for a short time after supply of electric power to the electric heater is stopped.
Conversely, by taking the deterioration of the catalyzer into consideration, if the time for which the electric heater is supplied with electric power is determined such that the temperature of the catalyzer still rises to the activation temperature even though the catalyzer has deteriorated, the temperature of the catalyzer becomes considerably higher than the activation temperature before the supply of power to the electric heater is stopped, when the catalyzer is new. Accordingly, in this case, a problem arises in that unnecessary energy is used for heating the catalyzer, and further, the catalyzer may be deteriorated sooner due to the excessive heating operation by the electric heater.